As light propagates through a first transparent material having a first index of refraction and reaches an interface with a second transparent material having a second index of refraction, a portion of the light is transmitted through the interface and into the second material while a portion of the light is reflected away from the interface and back into the first material. The closer the angle of propagation of incident light is to the normal angle (i.e. perpendicular to the interface), the more light is transmitted through the interface and the less amount of light is reflected by the interface. As the angle of incident light deviates from normal, at some point it will reach an angle called the critical angle where all of the light is reflected back into the first material and none is transmitted through the interface and into the second material. This phenomenon is called total reflection, and is dependent, in part, on the difference between the refractive indices of the two materials. Where the refractive index difference between the two materials is great, the critical angle will be relatively small. Where the refractive index difference is small, the critical angle will be relatively large.
Various methods have been used to increase the transmission or reflectance of light between two mediums having different indices of refraction, including the use of reflective or anti-reflective coatings and similar methods.
Many processes exist for depositing thin films of materials on substrates for these purposes. One such process involves chemical vapor deposition of various species, such as metals and metal oxides. Other deposition methods include plasma spraying and combustion chemical vapor deposition. These methods make use of precursors that can be vaporized and then deposited on a substrate by some means such as decomposition on a hot substrate or deposition on a charged substrate.
The drawbacks to these methods are that they are expensive and occasionally poisonous starting materials are used. In many instances, the starting materials are air sensitive and must be handled under an inert atmosphere. An additional drawback of such methods is that the substrate for the deposition of the coating must often be placed under a vacuum, increasing the cost of the deposition and limiting the size of the substrate that can be treated.
Screen-printing techniques are also used to deposit modifying surface layers onto various substrates. Examples where screen-printing techniques can be used to deposit opaque layers on a substrate include the printing of black enamels onto automotive windshields, the printing of silver de-frost lines on the rear window (backlight) of automobiles, and the printing of precious metal preparations to decorate dinnerware. Screen-printing techniques can also be used for the manufacture of decals that can be later applied to glass or ceramic substrates, or printing enamels on large panes of glass for architectural applications. Although screen-printing techniques are well adapted for depositing layers on a substrate, they conventionally have the goal of depositing an opaque layer to give color or decorative effects.
Other coating methods useful for the deposition of opaque layers on the surface of a substrate are roll coating, curtain coating, band coating and spray coating techniques. These techniques can be used to apply coatings to glass and ceramic substrates, especially in the architectural and container glass markets.
Another coating technique used to apply metal oxide coatings is the sol gel process. In this method, metal salts such as nitrates, are dissolved into solutions. Water is used as a convenient solvent for such solutions. Complex-forming agents, such as organic acids, are added to the solutions. The water is dried from the solution to create a gel and the gel is then fired. The organic acid and the nitrate salt combust during the firing process to leave the oxide behind. In sol gel reactions, the metal atoms are mixed on an atomic scale and combustion occurs between the nitrate anions and the citric acid. The temperature needed for synthesis of a powder is therefore significantly less than that needed for a traditional solid-state reaction.
One example of the sol gel process includes the fusing of blue cobalt aluminate coatings to a silica glass. The gel is formed from aluminum nitrate, cobalt nitrate, and citric acid. All three starting materials are dissolved in water. The water solution is dried on the glass substrate and the substrate is fired to give a blue coating. To verify the identity of the powder, a portion of the gel can be dried and the powder heated under air in a differential scanning calorimeter. A small exothermic peak at 704° Celsius will indicate that cobalt aluminate is formed.